Various studies have been conducted on the use of moisture-curable polyurethane hot-melt adhesives, which are solvent-free, as environmentally compatible adhesives, particularly in the bonding of fibers and the lamination of construction materials, and they have been widely used in industry.
Recently, with the growing need for lighter and thinner optical elements, research has been directed to the lamination of optical elements using hot-melt adhesives instead of acrylic pressure-sensitive adhesives, which have been commonly used.
For example, an adhesive is disclosed that uses a moisture- and heat-resistant hot-melt adhesive composition containing (a) 100 parts by weight of a polyurethane resin having a flow onset temperature of 55° C. to 110° C., (b) 5 to 150 parts by weight of a saturated polyester resin having a Tg of 0° C. to 110° C. and a molecular weight of 10,000 to 25,000, (c) 10 to 150 parts by weight of an epoxy resin having a softening point of 60° C. to 140° C. and a molecular weight of 700 to 3,000, and (d) 10 to 200 parts by weight of an inorganic filler surface-treated with a coupling agent (see, for example, PTL 1).
This adhesive has a practical level of moisture and heat resistance. Unfortunately, this adhesive has insufficient water resistance; if a laminate formed using the adhesive is immersed in water, water may enter the laminate within a relatively short period of time.
Despite the advantage of low-temperature bonding, the above moisture- and heat-resistant hot-melt adhesive composition is impractical in situations where fast curing is desired. Another disadvantage is that this adhesive, which contains an epoxy resin, has insufficient flexibility and is therefore of limited use.